Ink jet printers have gained wide acceptance. These printers are described by W. J. Lloyd and H. T. Taub in "Ink Jet Devices," Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988) and by U.S. Pat. No. 4,490,728. Ink jet printers produce high quality print, are compact and portable, and print quickly but quietly because only ink strikes the paper. The major categories of ink jet printer technology include continuous ink jet, intermittent ink jet, and drop-on-demand ink jet. The drop-on-demand category can be further broken down into piezoelectric ink jet printers and thermal ink jet printers. Drop-on-demand ink jet printers produce drops by rapidly decreasing the volume of a small ink chamber to initiate a pressure wave that forces a single drop through the orifice. Capillary action causes the ink chamber to refill.
The typical ink jet printhead has an array of precisely formed orifices attached to an ink jet printhead substrate having an array of ink jet drop ejectors that receive liquid ink (i.e., colorants dissolved or dispersed in a solvent) from an ink reservoir. In thermal ink jet printheads, each ink jet drop ejector has a thin-film resistor, known as a heater, located near or opposite from the orifice so ink can collect between it and the orifice. When electric printing pulses drive the heater, a thin layer of ink near the surface of the heater vaporizes and propels a drop of ink from the printhead. In piezoelectric ink jet printheads, each ink jet drop ejector has a piezoelectric transducer located near or opposite from orifice so ink can collect between it and the orifice. When electric printing pulses drive the piezoelectric transducer, a volumetric or elongational change occurs within the piezoelectric material that is mechanically coupled to the drop ejector in such a manner as to eject a drop of ink from the orifice. Drop ejection orifices are arranged in an array, typically in one or more columns, to achieve the desired vertical printing resolution. Properly sequencing the operation of the ink jet drop ejectors causes characters or images to form on the recording medium as the printhead scans across it.
The volume of ink drops ejected from ink-jet printers is temperature dependent because physical properties of the ink, such as surface tension and viscosity, depend on the ink temperature. Additionally, the energy available for bubble nucleation in thermal ink jet drop ejectors depends on temperature. This factor further contributes to the variation of drop volume with temperature. The temperature of the drops ejected from piezoelectric and thermal ink jet printheads substantially equals the temperature of the drop ejectors because the thermal capacity of the drop ejectors greatly exceeds that of the ink contained in them and because the ink contained in them dwells within them long enough to become in substantial thermal equilibrium with them.
Print quality is particularly sensitive to variations in the ink drop volume because these variations cause the spot size on the recording medium to vary and thereby affect the darkness of black-and-white text, the contrast of gray-scale images, and the chroma, hue, and lightness of color images. The chroma, hue, and lightness of a printed color depend on the volume of each subtractive primary color drop, namely the volumes of cyan, magenta, yellow, and black ink drops. If the volume of the ejected drops increases or decreases while a page is printed, as would happen if the printhead substantially heats up during this process, the colors at the top of the page may not match the colors at the bottom of the page.
Ink jet drop ejectors must eject drops over a wide range of operating temperatures. A drop ejector that creates satisfactory print when it is at room temperature may eject drops that are too large when it becomes hot. The excessive ink degrades the print quality by causing: the printed spot size to grow, the bleeding of ink spots having different colors, and, potentially, the cockling and curling of the paper.
Another problem occurs when drop ejectors become very warm. The dissolved gases in the ink diffuse out and form gas bubbles in the drop ejectors that can cause the drop ejectors to deprime. For example, consider a simple thermal ink jet printhead with three drop ejectors sharing a common heat conducting substrate. If drop ejector "one" and drop ejector "three" are printing at 100% duty cycle (i.e., every pixel at the maximum drop ejection rate), some of the heat they produce will flow into the silicon substrate and heat it. This substrate conducts heat to drop ejector "two" placed between "one" and "three". In extreme situations, where "two" does not eject any drops and the ink remains in the drop ejector, dissolved gases in the ink may come out of solution and deprime drop ejector "two" as a result of heating by drop ejectors "one" and "three". Furthermore, at high temperatures, the physical properties of the ink and the energy produced by the vaporization of ink in a thermal ink jet printhead may change to the extent that print quality becomes unsatisfactory. Therefore, management of printhead temperature under various environmental conditions and printhead duty cycles is an objective in the design of a thermal ink jet printing system: If the printer controller could measure the temperature of the drop ejectors, it could compensate for high temperatures by reducing the energy in the firing pulses and/or reducing the print speed and thereby cause the drop ejector to eject drops of nearly constant volume.
Previously known techniques for measuring the temperature of an ink jet drop ejector employ discrete devices such as thermistors and thermocouples. These devices have several disadvantages: their installation on the printhead substrate requires additional manufacturing steps and their large size prevents them from being located near the ink jet drop ejectors. This remote installation introduces a time lag in thermal measurements and inaccuracies in transient temperature measurements.
Another previously known technique for measuring the average temperature of an ink jet drop ejector substrate employs a thermally sensitive resistor (TSR) formed in the conductor layer of the printhead substrate around the ink jet drop ejectors. One disadvantage of a TSR is that it adversely affects thin film production yields because achieving control limits on the nominal resistance and coefficient of resistivity requires the rejection of some devices. Another disadvantage is that the TSR measures the average temperature over the entire printhead substrate instead of the temperature of an individual ink drop ejector.
Further disadvantages of discrete temperature sensors and TSR's include the addition of analog devices to each printhead and the calibration they require that adds to the cost and complexity of the printer. After combining the tolerances of the various analog components with the limitations on accuracy mentioned earlier (e.g., the significant distance between the temperature sensors and the ink drop ejector and the measurement of the average temperature of the printhead substrate instead of the temperature of a particular ink drop ejector), the uncertainty of the temperature measurements maybe a significant fraction of the operating range of the printhead, This can result in ineffective printhead thermal management producing unnecessary constraints on throughput or inadequate control of print quality parameters.
For the reasons previously discussed, it would be advantageous to accurately and inexpensively measure the temperature of individual drop ejectors, so that the printer can minimize variations in the ejected drop volume.
The present invention is a method and apparatus for measuring the temperature of individual ink jet drop ejectors by measuring the temperature of their ejected drops. A printhead is positioned so that drops ejected from it strike a temperature sensor. The ink jet drop ejector ejects several hundred drops to the temperature sensor and it measures the temperature of these drops which thereby measures the temperature of the ink jet drop ejector. The temperature sensor has a low heat capacity that enables it to respond quickly to the temperature of the ejected ink drops. An ink drop collection chamber surrounds the temperature sensor and collects the ejected ink drops that cover the temperature sensor. Also, the present invention has a capillary bundle that wicks accumulated ink from the temperature sensor to a waste ink accumulator. The temperature sensor, ink drop collection chamber, capillary bundle, and waste ink accumulator can be part of a printhead service station (which performs capping, wiping, priming, and other functions) or a stand-alone component within the printer.
An advantage of the present invention is that it measures the temperature of each individual drop ejector during operation. This is important because the temperature of each individual ink jet drop ejector affects the volume of the ejected drops it produces and the consistency of this volume influences the quality of the recorded image.
Another advantage of the present invention is that it facilitates improved printhead thermal management. Once the temperature of each individual drop ejector is known, high temperatures can be reduced by slowing down the print speed, by printing with every other drop ejector, by not using a drop ejector that is too warm, by driving the drop ejector with lower energy pulses, and other means that reduce the amount of energy transmitted to that drop ejector until it cools down. Thus, the present invention allows better thermal management of individual drop ejectors.
Another advantage of the present invention is that it does not require the addition of hardware to the printhead substrate that reduces the production yields of the ink jet printhead chips and requires extra space on the ink jet printhead substrate. This feature makes the present invention inexpensive and simplifies its implementation into existing designs. Furthermore, this invention does not require separate analog electronics for each printhead substrate and a calibration procedure that requires a reference temperature measurement.